This disclosure relates to linear amplifiers having level shift function with small circuit areas and low power consumptions. The linear amplifiers may also have variable gain and low-pass filter functions.
Circuits such as shown in FIG. 8 that perform level shifts while keeping linearity of analog signals are known. See, for example, Japanese patent publication JP 2001-244760. In the level shift circuit shown in FIG. 6, the input voltage signal Vin is an output signal of the preceding stage circuit that operates with a first power supply voltage VDD1. The following stage circuit, which includes the voltage amplifier or an operational amplifier 11, operates with a second power supply voltage VDD2, which is different from VDD1.
The input signal Vin is input to an inverting input terminal of the voltage amplifier 11 through an input resistance R1. Moreover, a reference voltage Vref is input to a non-inverting input terminal, and the output signal Vout is fed back to the inverting input terminal of the voltage amplifier through a feedback resistance R2. As a result, assuming that the gain of the voltage amplifier 11 is sufficiently large, the voltage at the inverting input terminal becomes equal to the reference voltage Vref. Accordingly, a current Ib defined by the following equation flows through the input resistance R1.Ib=(Vin−Vref)/R1The current Ib also flows through the feedback resistance R2, and a voltage of the output signal voltage Vout is defined as follows.
                    Vout        =                ⁢                  Vref          -                      R            ⁢                                                  ⁢                          2              ·              Ib                                                              =                ⁢                  Vref          -                      [                          R              ⁢                                                          ⁢                              2                ·                                                      (                                          Vin                      -                      Vref                                        )                                    /                  R                                            ⁢                                                          ⁢              1                        ]                              As a result, an average voltage, or a common mode voltage, of the output signal Vout can be controlled by the common mode voltage of the input signal Vin and the reference voltage Vref. That is, a level shift, or a level down, of the signal is accomplished.
Consider, for example, the following case: a level shift from a signal in a first region with 3.3 V supply voltage to a signal in a second region with 1.2 V supply voltage, the reference voltage Vref=1.0 V, the input common mode voltage Vin_common=1.5V, and the resistances R1=R2=500 Ω
In this case, an average of the current, or DC current, flowing through the resistances R1 and R2 is 1 mA, and the output common mode voltage Vout_common becomes 0.5 V. That is, a −1.0 V level shift is performed. Assuming that the gain of the voltage amplifier is sufficiently large, the gain Av of the circuit is defined as follows.Av=R2/R1=1Thus, amplitude of the input signal Vin is maintained in the output signal Vout.
It would be easy to understand the operation of the linear amplifier shown in FIG. 6 by imagining a movement of see-saw with arms having lengths of R1 and R2, respectively, from the fulcrum. See FIG. 7. When an input signal Vin at the end of the input resistance R1 moves around a higher common mode voltage, an output signal Vout at the end of the feedback resistance R2 moves around a lower common mode voltage Vout_common.
It is also possible to generate an output signal with a higher common mode voltage from an input signal with a lower common mode voltage. In this case, DC current flows toward the input signal side Vin through the resistances R1 and R2.
FIG. 9 shows another known level shift circuit disclosed in Japanese patent publication JP-2002-344258. The circuit shown in FIG. 9 includes, in addition to the same circuit components constituting the circuit shown in FIG. 6, a constant current source 23 connected between the inverting input terminal of the voltage amplifier 11 and the ground. The constant current source 23 draws a current Ic.
The circuit shown in FIG. 9 operates same as the circuit shown in FIG. 6 when the constant current source 23 is not connected. On the other hand, when the constant current source 23 is connected and if Vin_common=Vref, the current Ic flows in the feedback resistance R2. In this case, the output common mode voltage Vout_common is determined as follows.Vout_common=Vref+Ic·R2Thus, the level shift circuit shown in FIG. 9 performs level shift, or level up, to generate output signal Vout from input signal Vin by using the reference voltage Vref.
In the level shift circuit shown in FIG. 6, however, it is required to generate a reference voltage Vref appropriate for the input common mode voltage Vin_common and the output common mode voltage Vout_common. Further, the input resistance R1 and the reference voltage Vref determine the additional current Ib, which is required to drive the voltage amplifier 11. Accordingly, it becomes necessary to modify the construction of voltage amplifier 11 when one of the input resistance R1 and the reference voltage Vref varies.
Further, in the level shift circuit shown in FIG. 6, it is difficult to increase the amount of level shift because the reference voltage is used as a fulcrum. As an example, consider a case of level shift from an input signal in the 3.3 V supply voltage region with the common mode voltage of 2.5 V to an output signal in the 1.2 V supply voltage region. If the voltage amplifier 11 is placed in the 1.2 V supply voltage region, the reference voltage Vref cannot become higher than 1.2 V. As a result, the common mode voltage of the output signal Vout_common is equal to or lower than −0.1V, which cannot be realized conventionally.
Accordingly, it is necessary to construct the level shift circuit with a voltage amplifier placed in the 3.3 V supply voltage region. When the reference voltage Vref is set to 1.5 V, for example, the output common mode voltage Vout_common becomes 0.5 V.
However, such solution has some drawbacks. For example, the voltage amplifier in the 3.3 V supply voltage region may increase the power consumption. Further, the output stage of the voltage amplifier in the 3.3 V supply voltage region cannot operate when the required output common mode voltage Vout_common becomes too low. Accordingly, two stages of level shift circuits in both the 3.3 V and 1.2V supply voltage regions may be required.
The two stage level shift may become unnecessary if the ratio of the resistances R1 and R2 is carefully selected. In this case, however, the amplitude of the signal decreases because the total gain of the linear amplifier is defined by the ratio of R2/R1.
The reference voltage Vref may be generated from the input common mode voltage Vin_common and the output common mode voltage Vout_common. For example, as shown in FIG. 8, the difference between the input common mode voltage Vin_common and the output common mode voltage Vout_common, which are buffered by respective voltage followers constructed with voltage amplifiers 12 and 13, may be divided by resistances R21 and R22. Here, the ratio between the resistances R21 and R22 may be set as R21:R22=R1:R2.
Moreover, the level shift circuits shown in FIGS. 6 and 9 cannot directly determine the output common mode voltage Vout_common. In the circuit shown in FIG. 6, the combination of the common mode voltage of the input signal Vin_common, the reference voltage Vref, the input resistance R1, and the feedback resistance R2 determines the output common mode voltage Vout_common. In the circuit shown in FIG. 9, the combination of the common mode voltage of the input signal Vin_common, the current Ic of the constant current source 23, and the feedback resistance R2 determines the output common mode voltage Vout_common.
That is, in neither of these circuits, the output common mode voltage Vout_common is determined by a single parameter. Furthermore, when the level shift circuit shown in FIG. 9 is constructed in a semiconductor integrated circuit, the feedback resistance R2 inevitably has a large variation. As a result, the output common mode voltage Vout_common also has a large variation. In other words, the output common mode voltage cannot be determined precisely.